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| Subject: | GISList: DEM vs. DTM (SUM) |
| Date: |
04/07/2003 07:55:01 AM |
| From: |
Jeff_Boudrie .. monroemi.org |
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Thank you to all that responded to my questions.
Here's the sum.
Jeff
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Hey,
Generally, these are interchangeable terms. They often are interchanged.
In my experience though, the DEM will refer to a continuous surface of
elevations represented in
the raster data model. The DTM will be the same surface, generally
represented by a very dense
network of point elevations. In this case the DTM will have been collected
by lidar or other
photogrammetric methods. Then the DTM would be used to create contours, or
a DEM, or a TIN.
However, a DTM could be created from a DEM, although it would not be as
accurate (interpolation
would be involved).
Often, a DTM is a by-product of the aerial imagery processing process. I
am not sure what one is
preferred, as they are interchangeable. This may be aerial-vendor
specific.
I think that lidar would be the preferred method of DTM collection. It can
see through tree
canopies, and get a very accurate measurement of the elevation of the
ground. I don't think it is
that costly, especially compared to collecting elevations through
photogrammetry. That is very
labour intensive, and quite expensive. However, it may be efficient to get
elevations at the same
time as aerial photography, if you are also getting aerials done, and are
not interested simply in
elevations.
Dave
-----------------
To the best of my understanding, they are synonomous.
David Irving
Technical Systems Analyst
Innovation Team
----------------------
Jeff,
As I understand it, DEM (Digital Elevation Model) and DTM (Digital Terrain
Model) are different terms for the same thing (see Burrough &
McDonnell, "Principles of Geographical Information Systems", 1998, for
example). Both refer to an interpolated, continuous surface representing
the
ground surface. Two types of DEM (DTM) are regular grids and triangular
irregular networks (TIN's). Many people refer to DEM = grid and DTM = TIN,
but
I think this is not strictly correct.
Yes, a DEM is required for orthorectification of aerial imagery. An
orthorectified image shows features in their true planimetric positions.
Ground elevation causes a diplacement in the position of features in an air
photo, which needs to be corrected for to get an orthorectified product:
the
DEM is used for this correction.
I'll leave comments on LIDAR alternatives to others more knowledgeable than
me! But I suspect if you want a very detailed DEM comparable to something
produced from LIDAR, alternatives will be similarly expensive.
sarah
email. northms@telus.net
----------------
Jeff,
For your first question here is the answer I have
Question : What is the difference between a DEM and a DTM?
Ans:
Digital Elevation Model (DEM), is a spatial dataset that contains a series
of elevation values.
A Digital Terrain Model (DTM) is a computer application that models the
lie-of-the-land.
DEMs may be:
1) arranged in a grid that covers the area of interest.
2) an irregular array of points that reflect the highs and lows of the area
of interest.
3) a set of points lying along contours (and thus of equal heights).
A DTM can use the DEM to calculate derived values like slope, aspect etc.
Regards
Kiran
---------------
hi ,
DEM and DTM are used sunonymously in earlier days. But
there are conflicting usage of these terminology. I
could define these two in these ways
DEM - Digital Elevation model can be referred to the
continous representation of change in relief of
terrain (height variations of terrains). Here the
third dimension is the elevation (X,Y,Z)
DTM - digital Terrain model can be defined as a
representation of surface other than the elevations.
ie here the 3rd dimension can be other than the
elevation like rainfall intensity, population density
etc.,
that's whay for GIS personnel, if the 3rd dimension is
the elevation, it is DEM and if the 3rd dimension is
other than the elevation it can be referred as DTM.
For the photogrammetrists (who normally works with the
topography and not the attributes associated with it),
the 3rd dimension will always be the elevation. that's
the reason DEM/DTM is used synonymously.
But now, there are few slight differences been brought
into this for the differentiation of the same.
1. When u say a DEM, it is always a regualr gridded
pattern (referred as 10 x 10 m DEM). If the same DEM,
when supplemented with the breaklines (which indicates
the sudden steep changes in the terrain surface) and
spotheights which still give the best approximations
of the terrain surface can be termed as DTM.
This DTM can contain, the bridge, flyover
representaion also.
As far as LIDAR is concerned, LIDAR as a raw data
gives the complete surface DEM or as it is termed by
LIDAR
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